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2023-2024 Vesicular Stomatitis Virus (VSV) 

Situation Report – January 22, 2024

Information current as of 8:00 am MST, 1/22/2024

Since the last situation report (1/9/2024), there have been no new VSV-affected premises 

identified in the United States. All confirmed cases in the 2023-2024 outbreak were vesicular 

stomatitis New Jersey virus (VSNJV) serotype. Updates are as follows: 

California

• No new suspect or confirmed positive premises identified since the last report

• Since the last situation report (1/9/2024), the following previously confirmed positive 

premises were released from quarantine: 2 premises in Sacramento County. 

• The last VSV quarantines in California were released on January 18, 2024. 

 

Classification of Cases 

Premises that have diagnostic laboratory confirmation of VSV are categorized as confirmed 

positive premises. Once a county is confirmed as VSV-positive, new equine premises presenting 

with clinical signs of VSV in that county are not required to be tested for confirmation of the 

disease, but the premises will be quarantined and classified as a suspect premises. Premises may 

also be classified as suspect if clinical animals on the premises fail to meet confirmed case 

definition, but there is diagnostic laboratory evidence of recent VSV infection. 

Quarantines

Confirmed positive and suspect premises are quarantined for at least 14 days from the onset of 

lesions in the last affected animal on the premises. 

Epidemiology and Disease Control

Vesicular stomatitis virus circulates annually between livestock and insect vectors in southern 

Mexico and only occasionally results in incursion to the U.S. when climatic and ecological 

factors support movement of VSV-infected insect vectors northward. Known competent vectors 

for transmission of VSV include black flies, sand flies, and biting midges (Culicoides spp.), but 

other insect vectors may also be involved. Susceptible species are primarily equids (horses, 

donkeys, mules) and cattle, but can also include camelids (llamas, alpacas), swine, sheep, and 

goats. Control measures on VSV suspect and confirmed positive premises include quarantine of 

all susceptible species, isolation of lesioned animals, enhanced biosecurity, and vector mitigation 

measures to reduce within-herd spread. 

Summary of the Outbreak (New information in blue)

The 2023 VSV outbreak began on May 17, 2023, when the National Veterinary Services 

Laboratories (NVSL) in Ames, Iowa, confirmed the first VSV-positive premises in San Diego 

County, California. Texas was subsequently confirmed VSV-positive by NVSL on June 15, 

Animal and Plant 

Health Inspection 

Service

Veterinary 

Services
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2023 (Maverick County, Texas) and Nevada was confirmed VSV-positive by NVSL on July 27, 

2023 (White Pine County, Nevada). 

Since the start of the outbreak, 319 VSV-affected premises were identified (99 confirmed 

positive, 220 suspect) in 3 states, California, Nevada, and Texas. Three hundred nine (309) of 

these premises had only equine species clinically affected, seven (7) premises had only clinically 

affected cattle (Fresno County, San Diego County, and Santa Barbara County, California), two

(2) premises had both equine and cattle clinically affected (Fresno County and Mariposa County, 

California), and one (1) premises had clinically affected rhinoceros (San Diego County, 

California). 

California identified 316 affected premises (96 confirmed positive, 220 suspect) in 19 counties 

(Fresno, Inyo, Kings, Los Angeles, Madera, Mariposa, Merced, Orange, Placer, Riverside, 

Sacramento, San Bernardino, San Diego, San Joaquin, San Luis Obispo, Santa Barbara, Tulare, 

Tuolumne, and Ventura Counties). Texas identified 2 affected premises (2 confirmed positive) 

in 2 counties (Maverick and Shackelford Counties). Nevada identified 1 affected premises (1 

confirmed positive) in 1 county (White Pine County, Nevada). 

All VSV-affected premises were released from quarantine in Nevada on August 9, 2023, Texas 

on August 22, 2023, and California on January 18, 2024. 

Summary and Status of VSV-Affected Premises

Table 1. Location, number, and status of VSV-affected premises. New information in blue. 

CALIFORNIA

Confirmed 

Positive 

Premises

Suspect 

Premises

Premises 

Released from 

Quarantine

Current 

Premises 

Quarantined

Fresno County 

Inyo County 

Kings County 

Los Angeles County 

Madera County 

Mariposa County 

Merced County

Orange County 

Placer County 

Riverside County 

Sacramento County 

San Bernardino County 

San Diego County 

San Joaquin County 

San Luis Obispo County 

Santa Barbara County 

Tulare County 

Tuolumne County 

Ventura County 

10 

1 

1 

3 

2 

3 

1 

1 

3 

9 

3 

3 

26 

1 

4 

9 

5 

2 

9

49 

2 

0 

4 

21 

8 

0 

0 

0 

36 

0 

7 

42 

0 

6 

4 

4 

3 

34 

59 

3 

1 

7 

23 

11 

1 

1 

3 

45 

3 

10 

68 

1 

10 

13 

9 

5 

43 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

TOTAL: 19 COUNTIES 96 220 316 0
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Map 1 (Current). Counties with Current VSV-Quarantined Premises

There are no counties with premises currently quarantined for VSV in the United States. 

Table 2. Cumulative count of species clinically affected on VSV-quarantined premises. 

State Premises with 

clinically 

affected equine 

species

Premises with 

clinically 

affected bovine 

species

Premises with 

clinical signs in 

both equine 

and bovine 

species 

Premises with 

clinical signs in 

other 

susceptible 

species

Total affected 

premises

CALIFORNIA 306 7 2 1* 316

NEVADA 1 0 0 0 1

TEXAS 2 0 0 0 2

TOTAL 309 7 2 1 319

*wildlife park premises with clinically affected and confirmed VSV-positive rhinoceros

NEVADA

Confirmed 

Positive 

Premises

Suspect 

Premises

Premises 

Released from 

Quarantine

Current 

Premises 

Quarantined

White Pine County 1 0 1 0

TOTAL: 1 COUNTY 1 0 1 0

TEXAS

Maverick County

Shackelford County 

1

1 

0

0 

1

1 

0

0 

TOTAL: 2 COUNTIES 2 0 2 0

TOTAL PREMISES 

ALL STATES

99 220 319 0
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Map 2 (Cumulative). Counties with Premises Quarantined for VSV in 2023-2024

Figure 1. Epidemic Curve of 2023 VSV Outbreak
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Table 3. Timeline of Events

Date Event

May 17, 2023 VSV index case for the U.S. confirmed at NVSL – San Diego County, 

California

May 19, 2023 New affected county confirmed – Riverside County, California

June 1, 2023 New affected county confirmed – San Bernardino County, California

June 5, 2023 New affected county confirmed – Orange County, California

June 8, 2023 New affected county confirmed – Los Angeles County, California

June 15, 2023 VSV index case for Texas confirmed at NVSL – Maverick County, Texas

June 21, 2023 New affected county confirmed – Ventura County, California

July 24, 2023 New affected counties confirmed – San Luis Obispo County and Santa 

Barbara County, California

July 27, 2023 VSV index case for Nevada confirmed at NVSL – White Pine County, 

Nevada

August 7, 2023 New affected county confirmed – Shackelford County, Texas

August 9, 2023 All VSV quarantines released in Nevada

August 14, 2023 New affected county confirmed – Fresno County, California

August 22, 2023 All VSV quarantines released in Texas

September 27, 2023 New affected county confirmed – Tulare County, California 

October 17, 2023 New affected county confirmed – Madera County, California

October 23, 2023 New affected county confirmed – Mariposa County, California

November 2, 2023 New affected county confirmed – Inyo County, California

November 9, 2023 New affected county confirmed – Tuolumne County, California

November 14, 2023 New affected county confirmed – Kings County, California

November 27, 2023 New affected county confirmed – Merced County, California

December 14, 2023 New affected county confirmed – San Joaquin County, California

December 15, 2023 New affected county confirmed – Placer County, California

December 22, 2023 New affected county confirmed – Sacramento County, California

January 18, 2024 All VSV quarantines released in California